I think you make a very valid point about 80s villains. It wasn't necessarily that Mumm-ra, or any other 80s villain, was truly scary, it's just that the fights were done in such a way that, win or lose, it didn't really affect their 'threat-level'

By having more complex stories in modern cartoons, it also causes us to judge the capability of modern villains, and characters, more harshly.

The other way is that the heroes are just to lucky and they let the bad guys escape or show mercy to them.